Project Summary The consequences of irradiation- and/or Cisplatin (IR/CisP)-induced neuronal toxicity, i.e., neurological function deficits, often irreversible and permanent, represent a daunting challenge when treating patients with cancer. The underlying mechanisms of toxicity remain poorly understood and the ability to selectively protect neuronal survival while not compromising tumor control following IR/CisP is lacking. This proposal aims to determine the mechanisms protecting neurons from IR/CisP-induced cytotoxicity, with the overarching goal to provide evidence supporting novel strategies to decrease their neurotoxicity, while maintaining therapy efficacy and improving patient quality of life. NAD+-dependent deacetylase sirtuin 2 (SIRT2), which is highly expressed in differentiated neurons, is involved in diverse cellular processes including metabolism, response to oxidative stress, and tumor suppression. Our preliminary study has discovered a novel signaling network that connects SIRT2 to transcription coupled- homologous recombination repair (TC-HRR) and -nucleotide excision repair (TC-NER) of DNA damage and neuronal cell resistance to IR/CisP-induced cytotoxicity. Furthermore, our data revealed that CSB, the key mediator for TC-HR/TC-NER, is directly deacetylated by SIRT2. Moreover, the cyclin-dependent kinase 5 (CDK5), which is involved in DNA damage signaling in neuron cells, phosphorylates and inhibits SIRT2 function in DNA repair and neuronal survival following IR/CisP. We hypothesize that SIRT2 activity, which is suppressed by CDK5-mediated phosphorylation, protects neurons against IR/CisP-induced DNA damage by enhancing CSB- directed TC-NER and TC-HRR, thereby attenuating neuronal cytotoxicity and neurological deficits. A series of in vitro and in vivo experiments are proposed to test this hypothesis: Aim 1 will determine whether CSB mediates SIRT2 promotion of TC-NER/TC-HRR and neuron survival following IR/CisP. Aim 2 will determine how CDK5 negatively regulates SIRT2 function in TC- NER/TC-HRR and neuron survival following IR/CisP. Aim 3 will test if pharmacologically targeting SIRT2 specifically attenuates neuronal deficits following IR/CisP-based cancer therapy. Results from these studies will provide insights into the biological role of SIRT2 and the molecular mechanisms regulating the repair of IR/CisP-induced DNA damage. We expect this study to lay the foundation for future research investigating the targeting of the CDK5/SIRT2-CSB signaling axis as a novel strategy to alleviate and/or prevent neurotoxicity in cancer patients who need IR/CisP therapy.